Lithium-ion batteries are widely used in portable consumer devices like portable computers, mobile phones, and video or photographic cameras. In addition, large-scale lithium batteries are an attractive battery technology for hybrid electric vehicles, plug-in electric vehicles, and fully electric vehicles that will have a growing future market share due to their improved fuel economy and lowered CO2 gas emission. The growing importance of renewable energy production requires large energy storage systems and large-scale lithium-ion batteries are considered as potential battery systems used in smart grids to compensate peak power consumption in houses or to store the energy produced in off-grid photovoltaic systems.
Graphite is used as the electrochemically active material in the negative electrode of lithium-ion batteries. The graphite crystallinity is required to obtain high reversible specific charges (reversible electrochemical capacity) up to a theoretical value of 372 Ah/kg of graphite. The electrochemical redox process generating the energy is based on the reversible electrochemical intercalation of lithium into the graphite structure. The theoretical reversible capacity corresponds to a stoichiometry of LiC6 of the stage-1 lithium-graphite intercalation compound formed in this intercalation process. During the charging process of the lithium-ion cell, lithium ions from the positive electrode containing materials such as LiCoxNiyMnzO2 where x+y+z=1 and having a layered structure, the LiMn2O4 with spinel structure, or LiFePO4 of olivine-type migrate through the electrolyte and are intercalated in the graphite negative electrode. During the discharge process, the lithium ions are deintercalated from the graphite and inserted in the structure of the positive electrode material.
Details about the lithium-ion battery technology and carbonaceous negative electrode materials are described in several reviews and monographs (see for example: P. Novak, D. Goers, M. E. Spahr, “Carbon Materials in Lithium-Ion Batteries”, in: Carbons for Electrochemical Energy Storage and Conversion Systems, F. Béguin, E. Frackowiak (Eds.), Chapter 7, p. 263-328, CRC Press, Boca Raton FI, USA, 2010; Lithium-Ion Batteries-Science and Technologies, M. Yoshio, R. J. Brodd, A. Kozawa (Eds.), Springer, New York, N.Y., 2009; Lithium Batteries-Science and Technology, G.-A. Nazri, G. Pistoia (Eds.), Kluwer Academic Publishers, Norwell, Mass., USA, 2004; Carbon Anodes for Lithium-Ion Batteries, in: New Carbon Based Materials for Electrochemical Energy Storage Systems, I. Barsukov, C. S. Johnson, J. E. Doninger, W. Z. Barsukov (Eds.), Chapter 3, Springer, Dordrecht, The Netherlands, 2006).
Similarly, isotropic carbon materials are also advantageous for graphite bipolar plates in PEM fuel cells. Bipolar plates in fuel cells are normally plagued by the low through-plane conductivity when flaky additives are used. A material with a higher isotropy improves the through-plane conductivity of the bipolar plate.
Furthermore, isotropic carbon materials are beneficial in current collector coatings for various battery systems in order to achieve a high through-plane conductivity.
State of the Art in Graphite Particle Shaping and Coating
The rounding of platelet-like graphite particles can be achieved by special mechanical treatments, typically of natural graphite, in ball mills, hammer mills, or by an autogenous grinding process. Usually, in these processes a large amount of fines or graphite dust is created that has to be separated from the rounded graphite product, causing a significant loss of graphite. In addition, the rounding of particle contours does not significantly change the anisotropic arrangement of the crystallites contained in the particles and introduces strain into the particles that can lead to swelling effects in lithium-ion batteries when this strain is released during cycling.
The coating of the graphite particles by an amorphous carbon layer at present is achieved in the industry mostly by mixing the graphite particles with coal tar pitch either in a mixing process in which the pitch is mixed either as dry powder, molten liquid, or dissolved in an organic solvent. Subsequently the dry graphite/pitch mixture is carbonized and subsequently calcined under inert gas conditions at temperatures around 1500° C. One major problem of this coating process is the impact of coal tar pitch or other pitch types on the environment and health as some of the polyaromatic organic pitch ingredients (“PAHs”) are considered highly toxic, carcinogenic, and/or mutagenic. Therefore, coal tar pitch is considered as a substance of very high concern in the European REACH regulation and requires a controlled use in existing manufacturing processes. New permissions for production processes involving coal tar pitch are usually not granted by state authorities in Europe. Newly developed production processes therefore require alternatives to pitch-based coating processes that so far do not appear to exist. Pitch alternatives like special polymers or other solid organic substances that result in high carbon yield during carbonization are significantly more expensive, may not lead to the same quality of carbon coating, or are of environmental or health concern as well.
Graphitized mesocarbon microbeads (MCMB) stands for an artificial graphitic coke with spherical particle shape. When heating coal tar pitch at about 450° C. solid spherical coke particles are formed in the melt. The spherical particles are extracted, oxidized at elevated temperatures in air, carbonized and finally graphitized, resulting in particles with a smooth spherical surface.
Fast charge and discharge performance is of key importance for lithium-ion batteries in several applications. Specifically, automotive lithium-ion batteries used in fully electric vehicles or in plug-in electric vehicles require high capacity graphite-based active materials in the negative electrode. The alignment of the anisotropic graphite platelets along the platelet planes in the electrode and the electrode pore structure is considered to be responsible for the limited lithium-ion diffusion in the porous graphite electrode. The limitations with regard to lithium-ion diffusion and solid state diffusion of lithium are often seen as a reason for the non-ideal performance characteristics of graphite electrodes at high current rates during charge and discharge. The diffusion limitation of such graphite electrodes do not only reduce the cell power and charging speed, but may also cause the plating of metallic lithium at the negative electrode surface during the charging of the cell at high current rates, which is considered as a major safety problem of lithium-ion batteries.
In commercial graphite negative electrode materials based on natural graphite, the platelet-like shape of graphite is often modified to a more spherical or rounded shape. Rounded carbon particle shape is normally achieved by special mechanical treatments. The mechanical treatments abrade the edges thereby rounding the particles and as a consequence increasing the fine fraction in the particle size distribution and creating create many surface defects that can lead to parasitic reactions in lithium-ion batteries. However, these mechanical treatments do not significantly change the anisotropic particle character, i.e. resulting particles may show rounded particle contours, but do not avoid the problems described above.
Isotropic hard carbons have historically been used due to their favorable lithium intercalation/de-intercalation curves for applications in which fast charge and discharge and low temperature performance is important. The reversible capacity of these hard carbons is, however, lower than for graphite.
The importance of an isotropic pore shape and low tortuosity has been demonstrated in positive electrodes, see D. E. Stephenson et al. J. Electrochem. Soc. 2011, 158 (7), A781.
Isotropic graphite particles can be made by agglomeration of smaller particles in a random or at least near random orientation. However, a problem with many agglomerated graphite particles is the inherent fragility of the particle morphology since these agglomerates are typically only held together by adhesion (mainly through van der Waals forces), which facilitates the integrity of the coating (if present) and their break-up into smaller particles, thereby resulting in a higher surface area. This instability is particularly relevant for material that undergoes mechanical treatments for example upon pressing the graphite material into a negative electrode of a lithium-ion battery. It is readily apparent that the breakage of assembled particles is problematic, not the least in view of the change of the particle characteristics.
Thus, it would be desirable to produce carbonaceous materials that allow producing electrodes exhibiting on the one hand desirable fast charge and discharge characteristics, high reversible capacity, and/or exhibiting mechanical stability, allowing the particles to maintain their morphology and surface properties, for example during the pressing process for preparing the electrodes.